In prior art relating to circuit board manufacture, e.g. in specification U.S. Pat. No. 5,261,950, a method is disclosed wherein a fine-grained copper oxide powder having a granular size of the order of 10 μm or less is used for the formation of a paste. The paste is formed from copper oxide powder, a binding agent, alloying elements and additives. From the paste, a blank of an electrically conductive pattern of desired shape is formed on the surface of a substrate. The blank on the surface of the substrate is reduced using e.g. hydrogen (i.e. metallized) and sintered using a high temperature to produce a continuous copper foil.
In prior-art methods of this type using copper oxide powder as a starting material, which is reduced and sintered to form a metallic copper foil, the aim is always that the copper foil should remain permanently fixed to the surface of the underlying substrate. For this purpose, the prior-art paste contains a glass additive, among other things, which forms a strong bond to the underlying ceramic substrate. Since the metallization and sintering are performed at a very high temperature, which usually has to be at least of the order of over 500° C. to ensure adequate sintering, the substrate must be made of a material having a good heat resistance, such as ceramic.
The prior-art method has the advantage that a copper foil, which has a good electric conductivity, can be formed by a fast and low-cost procedure from copper oxide, which is cheap and has the characteristics of an insulator. Copper oxide powder as a starting material is chemically stable, unlike e.g. pure copper powder, which is so active that it tends to oxidize. In addition, copper oxide is cheap. In the copper foil manufacturing process, no copper oxide is wasted, so the efficiency and yield are good.
The problem with the prior-art method is that it is not suited for use in the formation of a electrically conductive pattern in practical applications where an electrically conductive pattern formed from copper foil having good electrical conductivity is to be laminated onto a plastic or paper substrate. Such applications include e.g. inductive sensors and antennas for various electrotechnical applications. It is obvious that a plastic or paper substrate would be destroyed if it were exposed to high temperatures as mentioned above.
A special application, which uses thin conductive metal foil on a 20 plastic and/or paper substrate are transponders. The transponders refer in this application to products, which comprise a circuitry pattern and an integrated circuit on a chip. The circuitry pattern is located on a substrate, and the integrated circuit on the chip is electrically connected to the circuitry pattern.
The transponders can be activated e.g. by an external HF or UHF field, so they need no power source. The transponders can be used for the identification of objects (products, persons, animals, etc.) by using memory data stored in the integrated circuit on the chip. Identification takes place from a distance, which varies depending on the used technology and the effective regulations. For the identification, the antenna produces an electric current for the integrated circuit when in the field of a reader. The substrate of the transponder can be provided with an adhesive surface to allow it to be attached to an object. The transponders may be disposable, like those used on foodstuff and other consumer goods packages, being destroyed after use, or they may be designed for continuous use, such as those used in logistic applications, bankcards, personal identification cards or other ID applications.
A typical circuitry pattern of a transponder has a thickness of 5-50 μm. The printed circuitry pattern is generally produced by the silk-screen printing technique. Electrical conductivity is provided by using conductive powder, which may be produced from e.g. silver, copper and graphite. Besides printed antennas, antennas are nowadays also made from e.g. thin copper wires by coiling and flattening the wires so as to form a thin foil. Other production methods include evaporating and electrolytic or chemical precipitation. From a continuous copper foil, which may be produced by different methods, the unnecessary parts are etched away to create an antenna pattern. The portion to be etched away may well exceed 50%. As the removal of the extra metal requires separate work stages, efforts are continuously being made in the field to create an antenna that is as near net shape as possible already at an early stage of the manufacturing process.
There are also prior-art methods wherein an RF-ID antenna is formed by die cuffing from a thin metal foil. The problems are that in this manufacturing method the quality is poor and most of the metal foil is lost as waste.
Specification EP 0991014 discloses a photolithographic method of forming an RF-ID antenna from silver powder using various light sensitive films, at least one intermediate agent facilitating the processing and two types of conductive metallic powder. Silver is expensive, so the antenna produced by this method is also expensive.
Especially in an RF-ID application, there is the problem that RF-ID tag antennas produced by prior-art manufacturing methods are expensive, which is an impediment to their more widespread use in disposable applications, such as e.g. foodstuff packages.
Generally known is also a method for producing a thin continuous foil e.g. by an electrolytic precipitation process. This method involves a cathode drum rotating in an electrolysis vessel and an arc-shaped anode consisting of one or more parts in the bottom part of the vessel. An electrolyte is supplied between the anode and the cathode, and the aim is to precipitate a copper foil as continuous and uniform as possible onto the surface of the cathode drum. When the deposited foil rises above the electrolyte, it is detached from the cathode and passed on to a further treatment process. The method has been under development since the 1930's, and it is described e.g. in U.S. Pat. No. 2,044,415 and in published patent application US 2002/5363.